UI architecture umbrella: vision delivery gap, v1 primitives, Panel render, vertical bands, tabs #2478

Track the owner's UI architecture review as one umbrella, with dedicated issues where noted:

1. Design-vs-delivery gap: panel-vision.md is largely candidate/hypothetical (PanelSurface, Panel trait, seven modes, inter-panel event bus); shipped = state machine, identity newtypes, ViewContent variant, budgets, top/bottom band only. Left/right bands remain E_UI_UNAVAILABLE ([CTX-1341] fix(ui): fail closed for unwired left/right band slots (#2226) #2234). Audit and label every vision item as implemented/candidate/deferred in the doc itself.
2. Declarative v1 expressiveness: Text/Row/Column/List cannot express file trees, diffs, charts, rich text, or self-drawn controls. Define an extension strategy (compound primitives vs a safe drawing tier) without breaking the sandbox; coordinate with RFC: plugin drawing capability (separate from the Panel RFC) #2477 (drawing capability RFC).
3. Panel::render(&mut Frame) contradiction: the trait is marked "not a plugin API" yet would hand plugins a Frame/GPU handle. Remove or replace it with the declarative/drawing boundary; document why.
4. Missing vertical layout: top/bottom bands only; IDE-style left tree + right terminal + bottom status is impossible. Track through [CTX-1341] fix(ui): fail closed for unwired left/right band slots (#2226) #2234 and add a layout acceptance test for left/right + top/bottom combinations.
5. Tabs structure without interaction: LayoutNode::Stack is documented as tabs, but no user-facing tab create/switch/close/reorder. Either wire it or stop describing it as tabs; add UX and tests.

Also add explicit "not implemented" markers to candidate docs so users/reviewers cannot read vision as shipped behavior.

Priority: P2 | Area: panel | Labels: feat,area:panel,P2 | Milestone: v0.1.0 | Task: CTX-2467